The invention relates to a semiconductor device comprising an integrated optical guide, which has at least one rectilinear part and one curved part, and comprising means for obtaining confinement of the light in the guide in the curved part, which means include a groove provided along the edge of the guide in the region of the curve.
The invention further relates to a method of manufacturing such a device.
The invention is used in the manufacture of integrated optical devices, such as, for example, Mach-Zehnder modulators or optical switches, which comprise guides having different curvatures, each guide comprising successively rectilinear parts and curved parts.
It is known from the publication entitled "Probleme der Topographie Integriert-Optischer Schaltungen" by Karl-Heinz Tietgen in "2213-Frequenz Vol. 35 (1981), September No. 9, Berlin-Deutschland", p. 248, to manufacture buried guides having a curved part.
The buried guide is simply constituted by an index ribbon higher than that of the substrate, this ribbon being obtained by implantation of titanium into the substrate of LiNbO.sub.3 and therefore it is at one level with the upper flat surface of said substrate. Such a completely buried guide always exhibits high losses.
In order to reduce the losses by radial diffusion into the curved part of the guide, this document indicates the formation of a groove, for example by etching, which exactly follows the edge of the buried guide having a larger radius of curvature and has the same height as the latter. This etched groove permits of enlarging the index jump between the guide and the atmosphere outside the guide on the side of the edge of the guide having a larger radius of curvature.
By this method according to the cited document, radii of curvature can be attained of the order of 0.5 mm with losses not exceeding 3 dB.
This document also indicates that in the manufacture of such curved guides losses are caused by diffusion due to the roughness of the edges of the guide with respect to the external atmosphere and that these losses are larger as the index jump is larger: and that other problems also have to be taken into account, which are connected with this index jump, if the latter is large, such as reflection, radiation and conversion of the mode.
In order to reduce these disadvantages, the etched groove has at its ends coinciding with the beginning and the end of the bend or curved part of the guide a bottom rising with a slight inclination towards the upper surface of the substrate so that the buried guide is only laterally confined by the substrate of lower index in the rectilinear parts.
The optical losses by radiation in the curved guides on the other hand have also been considered and described in the publication of "Marcatili and Miller" in "Bell Syst. Techn. 48, 2161 (1969)".
When an optical wave reaches the curved part of an optical guide, this wave must be adapted. For this purpose, a part of the energy carried is then converted into radiative modes. The energy then radiates in a dispersive manner parallel to the plane of guidance.
These losses by radiation are a consequence of the fact that, in order to maintain the same phase speed in the curved part, outside the bend, the electromagnetic field would have to be displaced at a speed higher than the speed of the light in the atmosphere. In fact, in order that the wave front is preserved and is displaced according to wave planes at a constant angular speed, it is necessary that the tangential phase speed is proportional to the distance between the considered point and the center of curvature of the guide. There is a given distance D adjusted from the outer edge of the ribbon constituting the wave guide, beyond which the speed of propagation is lower than the phase speed necessary to preserve the wave front. Consequently, from this distance, the mode can no longer propagate and the light radiates in the substrate situated in the convex part of the curve.
The conversion of the guided mode into a radiation mode is very penalizing for the monomode guides when they have radii of curvature of insufficient length.
A formula established by Marcatili and published by T. Tamir in "Topics in Applied Physics--Vol. 7" entitled "Integrated Optics", p. 133, gives the critical radius of curvature R as a function of the lateral confinement D of the mode, or more precisely as a function of the lateral extent of the evanescent waves and also as a function of the wavelength .lambda. used.
The radiative losses are no longer negligible when the radius of curvature of the guide satisfies the following inequality: EQU r&lt;24.pi..sup.2 D.sup.3 /.lambda..sup.2.
According to this formula, the more the lateral confinement increases, the smaller the radius of curvature of the guide can be without increasing the losses caused towards the exterior of the curvature.
For monomode guides, it can be deduced from this formula that the critical radius is of the order of 10 mm. It results therefrom that for radii of curvature smaller than 20 mm the losses by radiation begin to become substantial.
According to this formula, the indication is derived that it is necessary to increase the lateral confinement at the level of the curved part of the guides, which is perfectly in conformity with the indication of the first cited document.
However, according to the first cited document indicating the prior art, this problem is difficult to solve for the monomode guides due to the fact that the losses by diffusion and the rate of mode conversion increase simultaneously with the index difference with the confinement atmosphere.
Thus, the problems subsist to manufacture integrated optical guides having curved parts of much smaller radius of curvature than in the cited publications, typically 500 .mu.m, and having very small losses both in the curved parts and in the rectilinear parts, especially smaller than 1 dB/cm.
These problems are solved by the means of the invention, which permit of obtaining such a confinement of the curved parts of the guides that the radiative losses are suppressed without increasing thereby the losses by diffusion or mode conversion, which means are moreover applied to a ribbon guide structure having much smaller losses in itself than the completely buried guide structure known from the prior art.